1. FIELD OF THE INVENTION
The present invention relates to washers for both male and female threaded fasteners, more particularly to a washer having an integral cantilever which provides for a precisely controlled amount of deflection at the cantilever, thereby eliminating deflection at the location where the washer bears against a mating surface, resulting in an absence of brinelling thereinto.
2. DESCRIPTION OF THE PRIOR ART
While the inherent strength and operational simplicity of male and female threaded fasteners have made their use universal, several problems attend their use. These problems include inability to deflect under varying load conditions, loosening during operation, hoop-spreading due to overtightening or excessive loading, and brinelling into the surface of the parts being joined.
Many solutions have been devised in the prior art to rectify, or at least mitigate, the aforesaid problems associated with threaded fasteners. Generally, these solutions teach a washer which is structured to interact with a threaded fastener to solve one or more of the aforesaid problems.
Examples of solutions to the problem of the inability to deflect under varying load conditions include the following. U.S. Pat. No. 1,352,918 to Rohbock discloses a pair of seat plates which are each provided with an annular flange creating a recessed cavity on its inner face. The seat plates further have a concavely-shaped portion for receiving a convexly-shaped portion of a bolt or nut. U.S. Pat. No. 1,961,470 to Winchester discloses a frusto-spherical cup-shaped washer which is rotatably connected to a nut at a conically-shaped seat. Each of these solutions suffer from the drawback that they impose columnar loading directly onto the mating surface when a force is imposed which is along the axis of the threaded fastener. As a result, the variation of loading conditions is accommodated by compression, the degree of which depends upon the modulus of the material. Compressive response to variations in loading conditions is not a true deflection response, and usually results in brinelling into the mating surface, as well as fatigue failure and unpredictable loosening of the threaded fastener.
Further examples of solutions to the problem of inability to deflect under varying load conditions include U.S. Pat. No. 1,412,502 to Andrioli and U.S. Pat. No. 3,135,154 to Zenzic which disclose washer and nut devices which accommodate transverse forces originating in the fastened parts, but are not capable of accommodating deflection along the axis of the threaded fastener.
The problem of loosening is frequently handled by common split and unsplit lock washers. Examples of other solutions contained in the prior art include: U.S. Pat. No. 3,062,252 to Jackman which discloses a cone-shaped lock washer that includes an abutment to prevent hoop-spreading of its associated nut; U.S. Pat. No. 3,144,803 to Briles which discloses a lock washer having a peripheral lip that is spaced from its associated nut and engages the nut as the nut is threaded onto a bolt; and U.S. Pat. No. 3,476,010 to Markey which discloses a lock washer having a curved cross section and a humped portion that are flattened as its associated nut is threaded onto a bolt. A variation of these devices is U.S. Pat. No. 3,742,808 to Trembley which discloses a self-sealing nut having a washer separated from its associated nut by a powdered Teflon sealant.
The problem of hoop-spreading arises because the portion of a nut closest to the mating surface of a part that is being fastened is subjected, as a result of a combined loading from each of the threads of the nut, to forces which tend to spread or enlarge its cross section. Indeed, this high tension loading at the first one or two threads of the nut results in extremely high forces being applied to the bolt. Thus, it is common experience that bolts generally fail at a location defined by the plane of the joined parts. An example of a solution to the stress loading on nuts is disclosed in U.S. Pat. No. 3,087,371 to Orner wherein a nut having a conical portion mates with a conical portion of a washer, the washer providing circumferential strength to the nut at the highest point of stress. Further, overloading is prevented by providing an abutment on the nut calibrated to abut the parts being joined before a predetermined stress loading is reached. U.S. Pat. No. 2,588,372 to Erb is a further example of a nut having a conical portion intended for mating with a conical portion of a washer in order to control hoop-stress on the nut. U.S. Pat. No. 2,552,004 to Erdman is directed to solving a problem associated with the tendency of a bolt to fail at the point of connection of its head to its shank. Erdman teaches a bolt shank concavely spreading to meet the bolt head and a convexly-shaped mating portion of a washer that is used to distribute loading to the bolt head from the parts being joined.
The problem of brinelling, in which a threaded fastener or its associated washer is caused to embed into the surface of a part being fastened together, has frequently been solved by utilizing larger area washers and alternative materials which are less subject to wear. For example, U.S. Pat. No. 3,212,387 to Madansky discloses a solution particularly directed to surface galling by use of a plastic captive screw washer which provides protection against injury to the surface of the part being joined by the head of a bolt.
While the aforesaid proposed solutions to the problem associated with threaded fasteners may be acceptable in many applications, they are generally inadequate when addressing the unique and particularly demanding conditions encountered in motor vehicle usage, in which stud bolts and stud nuts that are used to secure wheels onto hubs are continually subjected to severe vibration and wide ranges in load variation.
In order to secure an effective connection of a wheel to the hub in a manner that will support the vehicle and, in addition, withstand loading caused by impact of the wheel with road hazards, two generally accepted threaded fastener systems are in use: stud-piloted wheels and hub-piloted wheels.
Stud-piloted wheels utilize a plurality of apertures arranged in a circular pattern, each aperture having a conical surface. These apertures are aligned with threaded studs on the hub. Stud nuts, which are used to secure the wheel to the hub, have a conically-shaped portion that complementarily mates with the conical surface of the wheel apertures. In this system, the wheel is seated and centered by alignment of the threaded studs projecting through the wheel apertures as well as the interaction of the conical surfaces of each of the wheel apertures with the conical surface of a prospective stud that has been inserted therethrough. The complementary conical surfaces provide precise final alignment of the wheel in addition to providing clamping of the wheel onto the hub.
Stud-piloted wheels are used primarily in automotive applications. Clearly, the problem of hoop-spreading is solved by this threaded fastener system, but there still remains a substantial problem relating to deflection inability, loosening and brinelling. In this regard, U.S. Pat. No. 3,386,771 to Verierr et al discloses a washer having a portion of either conical or spherical shape that is structured to fit into a complementarily shaped aperture. The washer may also have a slit to facilitate alignment and proper fitting. The washer cooperates with a stud nut at a mutually flat surface. As an additional example, U.S. Pat. No. 2,844,409 to Eksergian discloses a stud nut having a convexly-shaped portion which is structured to mate with a concavely-shaped portion of a washer. The washer further has a convexly-shaped portion structured to mate with a tapered portion of the wheel stud aperture. The washer has a flange portion which assumes the clamping pressure when the stud nut is fastened onto the stud.
Hub-piloted wheels also utilize a plurality of apertures arranged in a circular pattern, but the apertures do not include a conical surface. Further, the stud nuts do not have a conically-shaped portion. Alignment is established by interaction between a center hole in the wheel with receptive ledges machined into the hub. Consequently, there is no need for conical surface cooperation between the wheel apertures and the stud nuts. Since the hub itself serves to align the wheel, the number of studs required to safely secure the wheel to the hub is greatly reduced, frequently in half. In the prior art, a preferred form of nut therefor has been a cone locknut with a fitted flat washer of the type disclosed in the aforesaid U.S. Patent to Erb.
As mentioned above, because of varying loads and vibrational conditions to which motor vehicle wheels are constantly subjected, the threaded fasteners used to secure the wheel onto the hub are subject, acutely, to all the aforesaid problems of deflection inability, loosening, hoop-spreading and brinelling.
Hub-piloted wheels are finding increasing favor among truck manufacturers because of improved economy and load bearing capacity as compared with stud-piloted wheels. However, hub-piloted wheels suffer even more from the effects of vibration and load variation than do stud-piloted wheels. In the prior art, it has been suggested to add additional studs, increase the size of the studs and nuts, increase the thickness of the wheel disk, and provide thicker, wider washers, in order to solve these problems. However, none of these suggestions prevent loosening and brinelling because of continual load variation and vibration, which causes the tightest of nuts to embed into the wheel disk and eventually work loose. Frequent tightening is required and this only leads to progressive brinelling.
Accordingly, there remains in the prior art the need to devise a threaded fastener system which is immune to the aforesaid problems associated with threaded fasteners, and which is particularly well suited for use in motor vehicle hub-piloted wheels.